Optical glass fibers and optical cables are not in general optically isotropic, but exhibit a small residual birefringence, which may be caused by a slight core ellipticity, or by bending and coiling of the fibers. This residual birefringence varies more or less statistically over the length of the fiber and can be described in simple models using a binomial distribution. The polarization modes of light split more and more from one fiber segment to the other, pass through the birefringent fiber segment as fast or slow natural waves and split again, forming new waves in the next fiber segment. A certain group propagation time (group delay) corresponds to each path; the distribution of all of these propagation times has a half-value width, which is proportional to the square root of the number of fiber segments, and therefore to the square Toot of the fiber length. This is one characteristic of the binomial distribution.
The limits of the distribution are determined by the paths in the fiber in which light has passed through each birefringent fiber segment in the fast polarization mode or in the slow polarization mode. In about the average propagation time, light has passed through approximately one-half of the fiber segments in the fast polarization mode and the other half in the slow polarization mode. The individual fiber segments are not identical; their length, relative delay, and axis orientation may vary.
The different propagation times of the polarization modes disperse an optical pulse injected into a glass fiber with arbitrary polarization, as it passes through the fiber. In particular, in the case of digital transmission in the 10 GHz range, polarization mode dispersion results in an interference factor which limits the maximum transmission rate and/or the size of the system due to the dispersion of an optical pulse. The dispersion of the polarization modes of conventional glass fibers limits the capacity of many optical systems such as high-speed transmission links or broad-band delay-line structures.
Optical compensation of polarization mode dispersion must retard the components of an optical pulse that were faster than the mean and speed up the components that were slower than the mean. In the case of statistically independent evenly distributed orientations of the birefringent axes within an optical transmission link, compensation cannot be successfull unless the pulse is made to return through the entire fiber with orthogonal polarization. However, this is not acceptable in telecommunication, since the undistorted signal would then appear at the input, rather than the output, of the fiber.
It has been found, however, that not all optical fibers and cables with a high polarization mode dispersion have a statistically independent evenly distributed orientation of the birefringent axes, since cables are bent in general on cable supports in a preferred direction, for example, vertically and horizontally. For such optical transmission links, the axis orientations of the individual birefringent fiber segments are predominantly horizontal or vertical, so that the transmission signal has two main polarization states P.sub.U,S and P.sub.U,L, which are orthogonal to each other. Due to statistical fluctuations, however, the transmitted signal also has polarization components other than P.sub.U,S or P.sub.U,L, but the distributions are concentrated around P.sub.U,S and P.sub.U,L.
A particularly large polarization mode dispersion can be caused by local ellipticity of the fiber core. Even such a birefringence can have a preferred direction in space and therefore can be compensated for. A single polarization mode passes through many fiber segments only in the fast or slow mode, without being significantly dispersed at the input. Usually both polarization modes P.sub.U,S and P.sub.U,L are injected at the input of the transmission link; these exhibit no crosstalk into the other polarization channel over the entire length of the cable and exit the cable as two separate pulses at different times. The propagation time difference of this pulse can be compensated for in principle.
Partial compensation of polarization dispersion is possible if two predominant polarization states P.sub.U,S and P.sub.U,L that are orthogonal to each other and have different group propagation times still exist, although some statistical pulse dispersion occurs.
U.S. Pat. No. 5,600,738 deals with the topic of polarization mode dispersion in optical devices, in particular in optical switches and coupling elements for optical fibers. These are preferably made of highly birefringent materials such as lithium niobate (LiNbO.sub.3) and result in a strong distortion of an optical pulse as a result of the different propagation times for the fast and slow polarization modes for any input polarization of the pulse. Without compensating the polarization mode dispersion, the total length of these elements within an optical transmission link is a limiting factor for their transmission capacity. U.S. Pat. No. 5,600,738 suggests that, in order to compensate for polarization mode dispersion, the optical element be formed by two or more substrates coupled together so that the polarization dispersion generated in one substrate is compensated for in the other one. For example, two birefringent pieces of crystal may be used for this purpose, coupled together so that the fast polarization state of one crystal is projected onto the slow polarization state of the other crystal and vice-versa, so that the combined element has an overall polarization dispersion approximately equal to zero. If the material is the same, the pieces of crystal preferably have the same length; if the materials are different, they must be selected so that the propagation time differences cancel each other. Although polarization dispersion due to birefringent coupling elements and/or switches between optical fibers or cables can be compensated for with the method suggested by U.S. Pat. No. 5,600,738, polarization dispersion due to the slight birefringent properties of the glass fibers within the transmission link cannot be eliminated with this method.
In order to compensate for the polarization dispersion of a transmission link, one can determine the two characteristic directions of polarization P.sub.U,S and P.sub.U,L at its output and measure the respective group propagation times, and hence the propagation time difference between the polarization components P.sub.U,S and P.sub.U,L Compensation of polarization dispersion can be achieved if the two characteristic polarization directions are split at the output of the transmission link using a polarizing beam splitter, with the different group propagation times being compensated for by introducing optical propagation time segments in one or both partial beam paths. Subsequently the partial beam paths are recombined using a polarizing coupler.
In J. Patscher, R. Eckardt, Electronics Letters, vol.33, no.13, p.1157, it is shown that polarization-maintaining fibers can be used for compensation. Polarization-maintaining fibers are strongly birefringent, usually due to an elliptical core. Thus a propagation time difference between the polarization components P.sub.S and P.sub.L of such a fiber compensating for the propagation time difference between the main polarization components P.sub.U,S and P.sub.U,L of the transmission link can be produced with a relatively short piece of the compensation fiber. To achieve this, the polarization mode dispersion (or propagation time difference) of the transmission link must be measured, the correct length of the compensation fiber calculated, the compensation fiber cut off and spliced in the proper orientation, so that the previously too fast polarization component is slowed down and vice-versa. The problem with this method is the relatively complex measurements needed to determine the propagation time difference and thus the correct fiber length, as well as to determine the main polarization states P.sub.U,S and P.sub.U,L of the transmission link to choose the correct orientation of the compensation fiber. Once the compensation fiber has been prepared, no further adjustment to correct for errors in the calculated orientation is possible.